* Add a variant for tests and parallelize builds
- Fix a bug with boost defaulting to cxxstd=98 when cxxstd=11 is the
minimum for hpx
- Disable tests by default and use a variant to enable them if requested
- Enable parallel builds: each process takes up to 1Gb when tests are
not enabled.
* Remove tests variant
- The variant doesn't change what gets installed. Testing can be
activated using `spack install --test=root hpx`
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